CD16, an immunoglobulin Fcgamma receptor, functions as the primary activating receptor on NK cells and mediates positive signals. Activation of CD16 upon antibody cross-linking results lysis of target cells. CD16 is also known to play a role in T cell development and function. In human, certain auto-reactive IgG immune complexes often results in symptoms of inflammatory response and autoimmune diseases, such as lupus and rheumatoid arthritis. Sequence analysis has indicated that the receptor belongs to the immunoglobulin receptor family. Their extracellular ligand binding domain consists of two immunoglobulin modules and their cytoplasmic domain interacts with ITAM containing receptor gamma or zeta chain. To date, there has not been a crystal structure available for the family of Fc receptors. As a result, the molecular mechanism governing the activation of Fc receptors remains largely unknown. NKG2A, with its cytoplasmic ITIM motif, defines a negative signaling receptor on NK cell surface. Studies on the Ly49 molecule, a murine analog of human NKG2A, have shown that these receptors recognize class I MHC molecules as their ligands and protect target cells expressing certain alleles of class I molecules from lysis. The functional form of the receptor consists of a covalent heterodimer of NKG2A and CD94. Both chains are members of type II transmembrane C-lectin family of receptors. One family of inhibitory receptors on NK cells, termed Killer Inhibitory Receptors have been identified recently on human NK cells to have immunoglobulin-like extracellular ligand binding domains. Members of this family have either two (p58 molecules) or three (p70 molecules) Ig-like domains. It has been shown by the work of Eric Long and others that the protective effect of KIR against cell lysis is directly through the binding and recognition of class I HLA molecules. Furthermore, these receptors display HLA allotype specificity, as defined by two allotype specific mAbs GL183 and EB6 and several residues in the region of amino acid 77-80 of the HLA heavy chain have been implemented to contribute the specificity. Our interest is to understand at the molecular level the process of a specific recognition of HLA molecules by p58 receptors using X-ray crystallography. We have constructed a bacteria expression system to express and purify a human soluble receptor, p58cl43. It binds to GL183 specific HLA-C allotypes. In collaboration with Dr. Andrew Brooks of our lab, we are in the process of identifying the appropriate peptide ligands of a HLA-Cw4 that capable to interact with this receptor. It is our goal to study the crystallization and structure determination of the complex between this p58 receptor and its HLA ligand.